Semiconductor nanostructures can be incorporated into a variety of electronic and optical devices. The electrical and optical properties of such nanostructures vary, depending on their composition, shape, and size. For example, size-tunable properties of semiconductor nanoparticles are, of great interest for applications such as light emitting diodes (LEDs), lasers, and biomedical labeling. Highly luminescent nanostructures are particularly desirable for such applications.
Quantum dots are nanometer-sized clusters that are generally comprised of a few hundred to several thousand atoms from Groups II-VI, III-V, and/or IV-IV. The physical dimensions of quantum dots are on the scale of the excitonic Bohr radius, a property that leads to a phenomenon called the quantum confinement effect. The quantum confinement effect leads to the ability to tune the optical and electronic properties of quantum dots—properties that are not observed in either bulk solids or the molecular level. Mushonga, P., et al., “Indium Phosphide-Based Semiconductor Nanocrystals and Their Applications,” J. Nanomaterials 2012:Article ID 869284 (2012).
Inorganic shell coatings on quantum dots are a universal approach to tailoring their electronic structure. Additionally, deposition of an inorganic shell can produce more robust particles by passivation of surface defects. Ziegler, J., et al., “Silica coated InP/ZnS Nanocrystals as Converter Material in White LEDs,” Adv. Mater. 20:4068-4073 (2008). For example, shells of wider band gap semiconductor materials such as ZnS can be deposited on a core with a narrower band gap—such as CdSe or InP—to afford structures in which excitons are confined within the core. This approach increases the probability of radiative recombination and makes it possible to synthesize very efficient quantum dots with quantum yields close to unity.
Core shell quantum dots that have a shell of a wider band gap semiconductor material deposited onto a core with a narrower band gap are still prone to degradation mechanisms—because a thin shell of less than a nanometer does not sufficiently suppress charge transfer to environmental agents. A thick shell coating of several nanometers would reduce the probability for tunneling, or exciton transfer and thus, it is believed that a thick shell coating would improve stability—a finding that has been demonstrated for the CdSe/CdS system.
Regardless of the composition of quantum dots, most quantum dots do not retain their originally high quantum yield after continuous exposure to excitation photons. Elaborate shelling engineering such, as the formation of multiple shells and thick shells—wherein the carrier wave functions in the core become distant from the surface of the quantum dot—have been effective in mitigating the photoinduced quantum dot deterioration.
To exploit the full potential of nanostructures in applications such as LEDs and displays, the nanostructures need to simultaneously meet five criteria: narrow and symmetric emission spectra, high photoluminescence (PL) quantum yields (QYs), high optical stability, eco-friendly materials, and low-cost methods for mass production. Most previous studies on highly emissive and color-tunable quantum dots have concentrated on materials containing cadmium, mercury, or lead. Wang, A., et al., “Bright, efficient, and color-stable violet ZnSe-based quantum dot light-emitting diodes,” Nanoscale 7:2951-2959 (2015). But, there are increasing concerns that toxic materials such as cadmium, mercury, or lead would pose serious threats to human health al d the environment and the European Union's Restriction of Hazardous Substances rules ban any consumer electronics containing more than trace amounts of these materials. Therefore, there is a need to produce materials that are free of cadmium, mercury, and lead for the production of LEDs and displays.
Cadmium-free quantum dots based on indium phosphide are inherently less stable than the prototypic cadmium selenide quantum dots. The higher valence and conduction band energy levels make InP quantum dots more susceptible to photooxidation by electron transfer from an excited quantum dot to oxygen, as well as more susceptible to photoluminescence quenching by electron-donating agents such as amines or thiols which can refill the hole states of excited quantum dots and thus suppress radiative recombination of excitons. Coating the InP core with a ZnSe and/or a ZnS shell to form a core/shell structure—for example, InP/ZnSe/ZnS—is a general approach to increase and tailor the optical properties of InP nanoparticles and also make InP quantum dots more stable when used in display applications. See, e.g., Chibli, H., et al., “Cytotoxicity of InP/ZnS quantum dots related to reactive oxygen species generation,” Nanoscale 3:2552-2559 (2011); Blackburn, J. L., et al., “Electron and Hole Transfer from Indium Phosphide Quantum Dots,” J. Phys. Chem. B 109:2625-2631 (2005); and Selmarten, D., et al., “Quenching of Semiconductor Quantum Dot Photoluminescence by a π-Conjugated Polymer,” J. Phys. Chem. B 109:15927-15933 (2005).
It is ubiquitously accepted that the formation of core/shell structures are necessary to utilize the luminescence properties of quantum dots. It is the state of art to grow a perfect shell material around the core. The obtained core/shell dots should be single crystal with spherical morphology. Such core/shell dots usually will provide high quantum yield (QY) and good stability under the harsh application conditions. There are three basic requirements necessary for the formation of an ideal core/shell structure: the core should have a narrow size distribution, a spherical morphology, and fewer interface defects. The ideal shelling material should have a wider band gap than that of the core material, and also should have a small crystal lattice mismatch with the core material.
As shown in FIG. 1, TEM images of quantum dots with an InP core and ZnSe and ZnS shells show quantum dots that have poor morphology: poor size distribution, sharp corners, and many crystal facets. This poor morphology indicates that the InP cores have not been uniformly covered by the shelling material. During the growth of a shell on a core nanoparticle, the crystal structure and morphology of the shell tends to originate with the structure and morphology of the core. Thus, the irregularity of the core may be carried to the resultant core/shell(s) nanoparticles.
Talapin, D. V., et al., “Etching of Collodial InP Nanocrystals with Fluorides: Photochemical Nature of the Process Resulting in High Photoluminescence Efficiency,” J. Phys. Chem. B 106:12659-12663 (2002) disclosed improving the photoluminescence efficiency of InP nanocrystals through the use of fluorine compounds. The process disclosed by Talapin utilized size-selective precipitation followed by treatment with HF in combination with illumination using a long, pass filter which allowed for the reproducible preparation of monodisperse fractions of InP nanocrystals whose band edge emissions were tunable from approximately 1.7 nm to 6.5 nm. The etching process used in Talapin effectively removed phosphorus dangling bonds. Unfortunately, as explained in Mushonga, P., et al., J. Nanomaterials 2012:Article ID 869284 (2012), pre-etching the InP cores before shell growth resulted in poor core/shell structures as the remaining fluorine blocked the surface of the quantum dots.
A need exists to find a synthetic method that improves the morphology of the core material for nanoparticles. The present invention provides methods applicable to producing quantum dots having substantially improved sphericity. The present invention focuses on the treatment of quantum dot cores using an acid etching and/or annealing treatment. The treatment results in cores with less surface defects and better morphology. After coating the treated cores with shelling material, highly luminescent spherical core/shell dots are formed—for example, ZnSe/ZnS, CaSe/ZnSSe/ZnS, and InP/ZnSeS/ZnS.